1. Field of the Invention
The invention is concerned with a fluid friction coupling comprising a first coupling part in the form of a housing and at least one second coupling part in the form of a hub rotatingly received within the first coupling part, a set of plates associated with each coupling part, each set of plates being non-rotatingly connected to its associated coupling part, the plates associated with the first coupling part being interleaved with those associated with the second coupling part in a main chamber defined in the interior of the first coupling part, and a viscous medium at least partially filling the portion of the interior of the first coupling part which is not occupied by the plates.
2. Description of Prior Art
From GB Patent Specification No. 2 207 492, it is known to provide a drive assembly in a four wheel drive vehicle with a fluid friction coupling which comprises a main chamber in which the plates are arranged and an additional chamber for receiving the viscous fluid. It is the purpose of the coupling to generate different torque characteristics in the two different directions of rotation, with a higher torque building up in the main direction of rotation and with no torque or only a negligible torque being generated in the other direction of rotation.
Fluid friction couplings are preferably used in motor vehicles so that, when different adhesion coefficients occur, there is generated a traction force for the axle not permanently driven or for the other wheel by eliminating the differential function. The fluid friction coupling always builds up a torque if there occurs a speed differential and thus a relative movement between the plates. With low speeds of rotation, i.e. at low speeds or a vehicle speed of 0, a rapid transfer of the coupling into the so-called "hump mode" of the coupling (in which the plates engage one another and 100 per cent torque transmission occurs) is desirable. In other words, the objective is to establish a rigid connection between the driving axle and the vehicle axle driven by it in order to ensure that the vehicle moves. However, at high vehicle speeds, such transmission would have a negative effect on the driving behaviour of the vehicle.